1. Field of the Invention
This invention relates generally to magnetic recording disk drives for use with patterned media, wherein each data bit is stored in a magnetically isolated block on the disk, and more particularly to such a disk drive with an improved clock for reading and writing the data.
2. Description of the Related Art
Magnetic recording disk drives with patterned magnetic recording media have been proposed to increase the data density. In patterned media, the magnetic material is patterned into small isolated blocks or islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional continuous media wherein a single “bit” may have multiple magnetic domains separated by domain walls. To produce the required magnetic isolation of the patterned blocks, the magnetic moment of the regions between the blocks must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the blocks. U.S. Pat. No. 5,820,769 is representative of various types of patterned media and their methods of fabrication. A description of magnetic recording systems with patterned media and their associated challenges is presented by R. L. White et al., “Patterned Media: A Viable Route to 50 Gbit/in2 and Up for Magnetic Recording?”, IEEE Transactions on Magnetics, Vol. 33, No. 1, January 1997, pp. 990-995.
In conventional magnetic recording disk drives, where the data bits are written on continuous (non-patterned) media, the read clock signal for reading the recorded data is driven by the data readback signal. The readback data detector includes a timing recovery circuit that generates a timing error signal that controls the read clock frequency. Because the data must be detected before the timing error signal can be generated, there is always some latency between the data signal and the corrected read clock signal.
Also in conventional disk drives with continuous media, there is no requirement to write to precise positions on the media since all of the media contains magnetic material. However, to write on patterned media using the conventional unsynchronized approach, the media must be patterned perfectly with a single accurate period, and the effective motor speed of the spindle supporting the disks must be highly stable. Together, the accuracy of the media patterning and the stability of the spindle speed have to be such that bits could be written over distances up to 1 mm with positioning accuracy of the bits to about 10 nm. U.S. Pat. No. 6,754,017 describes a magnetic recording disk drive with patterned media that uses a special pattern sensor that senses the data blocks before they pass beneath the write head and generates a write clock signal.
What is needed is a magnetic recording disk drive with patterned media that has an accurate clock signal for reading data from and writing data to the patterned data blocks.